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454 I. Digel 25.1 Temperature Sensing Biomolecules In addition to specificity and sensitivity, successful thermoresponse should be one that is reversible and controlled. Thus, complexity of thermosensing and thermoregulation on the organism level may reflect the demands to handle and fine-tune responses to an important environmental factor in a dynamic fashion. However, ultimately, it seems that basic and rather simple (bio) chemical processes are serve as primary sensory events and, for that purpose thermotropic changes in physico-chemical state of biological molecules appear highly suitable. While the information available is somewhat scant, the picture emerging shows that cells can use signals generated through changes in nucleic acid or protein conformation, or changes in membrane lipid behavior, as sensory devices. Bellow we make a short overview of temperature-sensing properties of most important groups of biological macro-molecules. It is worthy to note that probably even water alone could serve as a primitive temperature sensor. In the middle of the twentieth century Oppenheimer and Drost- Hansen [14] reported that a number of more or less abrupt changes in the properties of water and aqueous solutions occur when the temperature is increased from 0 to 60 ◦ C. These changes or “kinks” occured within a rather narrow temperature range (±2 ◦ C) near 15, 30, 45, and 60 ◦ C, respectively and most probably caused by changes in the hydrogen bond network of the water. The authors argued that the temperature-induced structural changes in water and aqueous solutions exert a direct influence on biological phenomena. In a later work W. Drost-Hansen [15] suggested some mechanisms how these structural changes happening with vicinal (adjacent to surfaces) water can affect the behavior or activity of biological systems. It was argued that optimal conditions for a complex physiological activity (such as, for instance, growth) will occur somewhere near the middle of the interval between two consecutive kinks. This issue has been discussed in literature very controversially and has not received wide recognition. 25.2 Membrane Lipids Fluidity The physical state of phospholipid membranes does change in response to temperature shifts in phase-transition manner [16], but the temperature-induced changes in real biological membranes are not sharp because many kinds of fatty acids and cholesterol-like molecules present, having different characteristic temperature points of phase transition. Thus, it would not be surprising if cells (even those of bacteria) could utilize the changes in membrane fluidity as a thermometer device, assisted by protein helpers, playing a role of switchers, “sharpening” the temperature response. Microorganisms counteract the membrane propensity to rigidify at lower temperature and are able to maintain a more-or-less constant degree of membrane fluidity (homeoviscous adaptation). The cyanobacterium Synecocystis responds to decreased temperature by increasing the cis-unsaturation
25 Primary Thermosensory Events in Cells 455 of membrane-lipid fatty acids through expressing acyl-lipid desaturases [17–19]. Lipid unsaturation would then restore membrane fluidity at the lower temperature. In B. subtilis, this lipid modification is initiated through the activity of a so-called “two-component regulatory system” consisting of the DesK and DesR proteins [17]. Prokaryotic two-component regulatory systems usually consist of protein pairs: a sensor kinase and a regulatory protein [20]. It appears that it is a combination of membrane physical state and protein conformation that is able to sense temperature and even to translate this sensing event into proper gene expression. However, sensing of temperature through direct alteration in nucleic acid conformation might be more efficient temperature-mediated mechanism of gene expression. 25.3 RNA and DNA Thermotropic Reactions Theoretically, RNA molecules have a strong potential as temperature sensors, in that they can form pronounced secondary and tertiary structures [21], and through their ability to form intermolecular RNA: RNA hybrids [22]. Both of these processes greatly depend on the formation of complementary base pairing, and consequently one would anticipate these to be dependent on environmental temperature. Indeed, messenger RNAs, apart from carrying their coding information for protein generation are also rapidly emerging as regulators of expression of the encoded message. With unique chemical and structural properties, sensory RNAs perform vital regulatory roles in gene expression by detecting changes in the cellular environment either alone or through interactions with small ligands [23, 24] and proteins [25, 26]. Regulatory RNA elements, “riboswitches”, have been reported recently, responding to intracellular signals by conformational changes. Riboswitches are conceptually divided into two parts: an aptamer and an expression platform. The aptamer directly binds the small molecule, and the expression platform undergoes structural changes in response to the changes in the aptamer. The expression platform is what regulates gene expression. Riboswitches demonstrate that naturally occurring RNA can specifically response on versatile physical and chemical stimuli, a capability that many previously believed was the domain of proteins or artificially constructed RNAs [27]. RNA thermometers operate at the post-transcriptional level to sense selectively the temperature and transduce a signal to the translation machinery via a conformational change. They have usually a highly structured 5 ′ -end that shields the ribosome binding site at physiological temperatures [28–31]. Changes in temperature are manifested by the liberation of the Shine–Dalgarno (SD) sequence, thereby facilitating ribosome binding and translation initiation. It is known that both in prokaryotic and eukaryotic cells the geometry and tension of DNA is highly dynamic and corresponds to its functional activity. In the bacterial cell, chromosome and plasmid DNAs are contained in a “twisted” superhelical conformation [32, 33], where the degree of supercoiling varies in response to
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Primary Thermosensory Events in Cells - Springer

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